With a major speech at the Democratic National Convention earlier this month, Governor Deval Patrick finally enjoyed the breakthrough moment on the national stage he had been actively seeking since the beginning of his second term.

Now, Patrick finds himself grappling with the kind of crisis that has the potential to taint his national image and keep him bogged down in the details of governing.

The revelation that thousands of criminal cases may have been compromised by a chemist in the state drug lab crushes any hope Patrick may have had of finishing his term unburdened by scandal.

Unlike the questions about patron­age and an early-morning car crash that have dogged Patrick’s lieutenant governor, the problems at the state drug lab speak directly to Patrick’s management of a critical public safety function.

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They have emerged in a period when Patrick has often been away from Beacon Hill, traveling the country to fire up Democrats on ­behalf of President Obama. He has another trip planned Oct. 6 to address the Minnesota Democratic-Farmer-Labor Party’s Founders’ Day dinner in St. Paul.

On Tuesday, Patrick defended his handling of the drug lab’s problems, blamed them on a “rogue chemist,” and pointed out that she was hired in 2003, indirectly tracing the crisis to Mitt Romney’s term as governor.

The drug lab, in Jamaica Plain, has long been hampered by complaints that it is understaffed and underfunded, with a months-long backlog. But Patrick said those ­issues, which came to his attention years ago, were not related to the current imbroglio.

“We’re dealing with, by all ­accounts, a rogue chemist who for many years, going back to 2003 or 2004, has not done her job, and that’s gone undetected for a long time,” Patrick said after touring an urban farm in ­Roxbury.

“I know there are those who want to say this has to do with the budget challenges of the last two years, but that’s just not borne out by the evidence,” he added.

Recently released e-mails show that employees at the lab have been documenting their concerns about understaffing since at least 2008. In those e-mails, they said that ­Patrick’s public health commissioner was unresponsive to their complaints.

On Monday, Patrick accepted the resignation of the commissioner, John Auerbach, who took responsibility for managers failing to adequately investigate and monitor problems at the lab.

On Tuesday, House lawmakers launched their own investigation of the drug lab, a process that could prove problematic for the Patrick ad­min­istration. One of the committees conducting the investigation has the power to subpoena state ­officials to testify in State House hearings.

Patrick has denied any interest in running for president in 2016, but the drug lab’s problems could bedevil him if he were to seek national office or be tapped for a Cabinet position in a second Obama ­administration.

Brendan Ryan, the governor’s spokesman, said Patrick was unaware of any problems involving the chemist for seven months after lab staff were first informed because they were slow to inform their superiors in the administration.

“I know there are those who want to say this has to do with the budget challenges of the last two years, but that’s just not borne out by the evidence.”

Governor Patrick, defending his response to the drug lab scandal

In addition, an initial investigation ordered by Auerbach failed to uncover the full extent of the problems, Ryan said.

Lab staff first became aware of problems with the chemist in June 2011, when she took 90 drug samples from the evidence room, without signing them out, as required by protocol, ­Ryan said. But it was six months before Auerbach was ­informed, in December 2011. He ­ordered his lawyer to investigate, but did not tell his boss, JudyAnn Bigby, who is a member of Patrick’s Cabinet.

It would be another month until Bigby was informed. She immediately told Patrick, in January, that the chemist was being investigated, Ryan said. But even then, Patrick and other officials believed the chemist’s problems were limited to the 90 samples.

Patrick informed the district attorney whose cases were ­involved. The following month, in February, the governor ­believed the matter had been settled when Auerbach’s internal investigation did not ­uncover any problems beyond those 90 samples.

The administration moved to fire the chemist, Annie Dookhan, who quit in March.

The full extent of Dookhan’s ­alleged tampering came to light in July, when State Police began a long-planned takeover of the lab from the Public Health Depart­ment. Patrick and the Legislature had ­ordered the takeover in hope it might solve some of the managerial problems that had been revealed years earlier.

As the takeover was underway, lab staff told State Police they had serious concerns about Dookhan’s work. The State Police then told the governor’s office and the attorney general, who ordered the State ­Police to investigate Dookhan again. A month later, the troopers finished their investigation with a dire conclusion: Dookhan may have compromised 60,000 drug samples and jeopardized 34,000 criminal cases.

Patrick said that is when he first learned Dookhan’s name. He ­ordered the State Police to shut the lab the following day, Aug. 30. Now, he is embroiled in fallout from the crisis, which seems unlikely to end for months. He has set up a “boiler room” of officials to create a comprehensive list of defendants who may have had their cases affected by tainted evidence.

“I’m glad it’s being dealt with now,” Patrick said, brushing aside a question about why he was not notified more quickly. “And I’m glad every­body who needs to be at the ­table is there.”

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